The present invention relates to an image forming device such as a copier, facsimile, printer or the like. More particularly, the invention relates to an image forming device wherein toner is caused to travel from a toner carrier toward an electrode disposed on the backside of an image receiving member and be deposited on the image receiving member thereby effecting image formation, with the toner deposition being controlled by a toner passage controller based on image signals.
In recent years, advances in the performance of personal computers and in network technology have brought about a strong demand for printers and copiers with high processing capabilities that can handle a large amount of documents including color documents. However, image forming devices that can output black and white and color documents with satisfactorily high quality and processing speed are still under development.
An image forming technology known as a xe2x80x9ctoner jet(trademark)xe2x80x9d method includes causing toner to fly onto an image receiving member such as a recording sheet or an image carrying belt by the action of an electric field.
Image forming devices of this type are disclosed, for example, in Japanese Published Examined Patent Application No. 44-26333, U.S. Pat. No. 3,689,935 (Japanese Published Examined Patent Application No. 60-20747), Japanese Published Unexamined Patent Application No. 9-500842. As one example of these devices, the one disclosed in Japanese Laid-open Patent Application No. 10-100780 will be described with reference to FIG. 17.
In FIG. 17, reference numeral 31 denotes a grounded toner carrier for conveying charged toner. A control blade 32 is provided for charging toner and adjusting same in one to three layers on the toner carrier 31. Reference numeral 33 denotes a supply roller for supplying toner to the toner carrier 31 and charging the toner. Reference numeral 34 denotes a toner passage controller, in which toner passage holes 35 are formed, surrounded by control electrodes 36. Voltage is applied from a control power source 37 to the control electrodes 36 in accordance with image signals. Reference numerals 38 and 39 represent a backside electrode and a power source for same, respectively. An image receiving member 40 such as recording paper is conveyed on the backside electrode 38.
With the supply roller 33 and the toner carrier 31 in operation, a uniform toner layer is formed on the toner carrier 31 by the control blade 32. In this state, voltage is applied to the backside electrode 38, and while the image receiving member 40 is moved, voltage is applied to the control electrodes 36 in synchronism therewith based on image signals, from the control power source 37, which is, for example, a driving IC or the like. Thereupon, the toner on the toner carrier 31 passes through the toner passage holes 35 in accordance with the image signals and adheres to the image receiving member 40, whereby a desired image is formed on the image receiving member 40.
In order to form a fine image of for example 600 pi (i.e., at a density of 600 dots per inch) on the entire surface of the image receiving member 40, it is necessary to line up the toner passage holes 35 in the toner passage controller 34 at a corresponding pitch. Since this is obviously not achieved in a single row, the toner passage holes 35 and the control electrodes 36 are lined up in a large number of rows (eight rows in the example), as shown in FIG. 18. The toner passage holes 35 and the control electrodes 36 are both circular, and connection electrodes leading to the control electrodes 36 extend toward both sides with respect to the moving direction of the toner carrier 31 so as to avoid interference between them. The connection electrodes are connected to the leads of corresponding driving ICs for outputting control voltage.
Such construction has the problem of high cost in view of the large number of toner passage holes 35 and corresponding driving ICs. Moreover, there is the problem known as white line noise (hereinafter referred to as xe2x80x9cWLNxe2x80x9d) wherein grey or white streaks are produced in the image, because most of the toner on the toner carrier 31 is consumed at the first row of the toner passage holes 35 and the toner density decreases in the direction in which the toner passage holes 35 are arranged, i.e., in the direction perpendicular to the moving direction of the toner carrier 31.
In view of these problems, it has been proposed to provide deflection electrodes 41a, 41b in addition to the control electrodes 36 around the toner passage holes 35 as shown in FIG. 19 so as to deflect the flying toner, thereby enabling a plurality of dots to be deposited through one toner passage hole 35 (xe2x80x9cNew multiplexing method makes TonerJet even more low cost manufacturingxe2x80x9d by Ove Larson, Journal of Electrophotography, Vol. 36, No. 2, p. 46-49, 1997).
Referring to FIGS. 19 and 20, below the control electrodes 36 shown in FIG. 20A, deflection electrodes 41a, 41b are arranged in pairs on right and left sides of the toner passage holes 35 as shown in FIG. 20B. Voltage is selectively applied to the deflection electrode 41a, 41b, or to both of them, so as to determine toner landing positions 42a to 42c as shown in FIG. 19B. Voltage application to the deflection electrode 41a, 41b, or to both of them is switched over as the image receiving member 40 is moved. Accordingly, the deflection electrodes 41a, 41b are arranged at an angle such that tan xcex8 is ⅓ (18.4) with respect to the center lines through the toner passage holes 35 as shown in FIG. 20B to compensate for the movement of the image receiving member 40. Voltage is applied first to the deflection electrode 41a or 41b which is offset toward the upstream side with respect to the travelling direction of the image receiving member 40.
With this construction, the pitch P with which the toner passage holes 35 are arranged is 254 xcexcm, ensuring sufficient aperture area of the toner passage holes 35, whereby the control of flying toner is performed reliably. The toner passage holes 35 need be provided only in two rows as shown in the drawings for forming an image of 600 dpi, leading to a considerable reduction in cost.
The device shown in FIG. 17 adopts a structure wherein the image receiving member 40 is a recording sheet or the like, on which an image is formed directly. In the case of forming a color image, it is difficult to synchronize the timing of image formation of various colors because of the variations in the conveyance of the recording paper, because of which image quality can decrease. Therefore, in some cases it is preferable to use an intermediate image carrying belt as the image receiving member 40 and to transfer the image formed thereon onto the recording paper or the like, as disclosed in Japanese Laid-open Patent Application No. 10-100780.
Referreing now to FIG. 21, reference numeral 43 denotes an endless image carrying belt serving as the image receiving member 40, made of a resin film of about 1010xcexa9/cm resistance into which a conductive filler has been dispersed, and wound around a pair of two rollers 44a, 44b. Reference numeral 45 denotes a pick-up roller for feeding recording sheets 46 one by one from a paper supply tray. Reference numeral 47 represents a timing roller for synchronizing the supplied recording paper 46 with an image position. Reference numeral 48 represents a transfer roller for transferring a toner image formed on the image carrying belt 43 onto the recording paper 46. The transfer roller 48 is pressed against the roller 44a with the image carrying belt 43 interposed therebetween and voltage is applied thereto for the transfer of images. Reference numeral 49 denotes a fixing device for fixing the toner image transferred onto the recording paper 46 by applying heat and pressure thereto.
In such image forming device wherein the toner passage holes 35 are provided in two rows as shown in FIG. 20, most of the toner is consumed at the toner passage holes 35 on the upstream side with respect to the moving direction of the toner carrier 31, so that toner held on the toner carrier 31 has been decreased by the time it reaches the toner passage holes 35 on the downstream side. Thus, the above-described problem of xe2x80x9cWLNxe2x80x9d can easily occur. This could be prevented if the diameter of the toner passage holes 35 is decreased to minimum so that the areas where toner on the toner carrier 31 is consumed do not interfere with each other. On the other hand, this would cause clogging of toner in the toner passage holes 35. Thus, with regard to the hole diameter of the toner passage holes 35, there is a trade-off between the prevention of WLN and of toner clogging.
In the image forming device described above, particularly when the toner passage holes 35 are arranged in two rows as shown in FIG. 20, with connection electrodes 36a extending toward the upstream side and with connection electrodes 36b extending toward the downstream side, the problem of xe2x80x9cWLNxe2x80x9d can occur for the following reason: The control voltage applied to the connection electrodes 36a from the driving ICs on the upstream side causes the toner held on the toner carrier 31 to fly repeatedly to and from the connection electrodes 36a. Thereby, part of the toner on the toner carrier 31 is gathered to the regions opposite the connection electrodes 36a. As a result, toner is passed in a greater amount through the toner passage holes 35 on the upstream side, and the amount of toner flying through the toner passage holes 35 on the downstream side is decreased accordingly.
This problem occurs even when the toner passage holes 35 are arranged in one row, if the connection electrodes extend alternately toward the upstream side and the downstream side.
Moreover, in the above-described image forming device wherein the toner carrier 31 is a roller, if the toner passage holes 35 are formed in a plurality of rows in the moving direction of the toner carrier 31, the distance between the toner carrier 31 and the toner passage holes 35 differs from row to row, leading to a difference in the toner amount deposited on the recording paper 46 from row to row, resulting in detrioration of image quality.
In light of the above-described problems of the prior art, it is an object of the present invention to provide an image forming device in which white line noise and toner clogging are eliminated.
It is another object of the invention to provide an image forming device in which white line noise caused by electric fields is eliminated.
It is yet another object of the invention to provide an image forming device in which deterioration of image quality due to differences in the amount of toner applied from row to row of toner passage holes is prevented.
An image forming device according to one aspect of the present invention includes a toner carrier for holding and conveying charged toner, and a toner passage controller including a plurality of holes through which toner is passed and control electrodes disposed surrounding the holes. The control of passing of the toner through the holes is effected by applying voltage to the control electrodes in accordance with image signals. The holes are formed oblong in shape, with the length along a direction in which the toner carrier moves being longer than the width orthogonal to the lengthwise direction. Thereby, the consumption areas of toner on the toner carrier do not interfere with each other and the problem of white line noise is eliminated. Since the open area of the holes is sufficiently secured by the large length, toner clogging is reliably prevented.
The toner passage holes may substantially be rectangular, but preferably be elliptic.
The holes should preferably have the dimensions specified as 0.65xe2x89xa6W/Lxe2x89xa60.90, where L is the length of the holes in a direction in which the toner carrier moves, and W is the width in a direction orthogonal to the lengthwise direction.
The width W should be more than 65% of the length L in order to secure density of dot image formed on the image receiving member, as well as to prevent toner clogging in the holes. If the width W is less than 65% of the length L, the aperture area of the holes will be too small in view of the limitation on enlargement of the length L, whereby it will become difficult to secure sufficient amount of toner, leading to a decrease in the dot image density and causing toner clogging. If the width W exceeds 90% of the length L, then the effects of preventing the problem of white line noise will not be achieved.
The width W and the length L are most preferably set as follows in consideration of dot image density and prevention of toner clogging: 0.70xe2x89xa6W/Lxe2x89xa60.80.
Further, provided that the mean particle size of toner used in the image forming device is about 6 to 15 xcexcm, the holes should have an open area S determined as follows:
5000 xcexcm2xe2x89xa6Sxe2x89xa67000 xcexcm2 
Wxe2x89xa770 xcexcm.
The holes through which toner passes should have a minimum curvature radius at the periphery thereof which is more than the mean particle size of the toner. In this way, toner build-up in the corners of the holes which leads to toner clogging is prevented. The average particle size of toner is about 6 to 15 xcexcm, and particularly about 8 xcexcm.
The holes may have an inner wall surface having a surface roughness which is less than the mean particle size of the additives added to the toner, so that toner can hardly adhere to the inner wall of the holes for preventing toner clogging. The mean particle size of the additives is usually about 0.1 to 0.5 xcexcm. Forming of the holes having such surface roughness can be achieved by a drilling process using an excimer laser or a punch. Alternatively, such holes may be formed by first drilling the holes using a YAG laser, CO2 laser or the like, and by performing surface treatment such as etching or the like.
The inner wall surface of the holes may be made of a material having low affinity with the toner, so that toner can hardly adhere to the inner wall of the holes for preventing toner clogging. Specifically, the inner wall of the holes can be coated with a fluoride-based resin or a silicon-based resin or the like.
The inner wall surface of the holes may be coated with a material having low affinity with the toner and a melting point which is lower than that of the material forming the toner passage controller, so that such coating can be provided easily by dipping or spraying.
The holes may be formed with minute protrusions at the peripheral edge thereof on the side of the toner carrier, so as to prevent toner depposited around the holes from falling into the holes and accumulating therein, clogging the holes.
Such minute protrusions may be simply provided by employing a material having high ductility for the toner passage controller and drilling the holes by punching, whereby burrs are created which can serve as such protrusions.
The holes are formed in an elongated shape, with a length thereof along a direction in which the toner carrier moves being larger than a width thereof along a direction orthogonal to the lengthwise direction, and further, the toner passage controller is given larger tension in the moving direction of the toner carrier than in the direction orthogonal thereto. Thereby, the rectangular or elliptic holes are prevented from being deformed by the tension applied to the toner passage controller. Deterioration of image quality caused by changes in the aperture area of the holes is thus prevented.
The control electrodes surrounding the holes may have a larger width in a direction along a longer diameter of the holes than a width thereof in a direction along a shorter diameter of the holes. In this way, toner is more readily collected to both longitudinal ends of the oblong holes while the toner amount on the lateral sides of the holes is decreased, whereby bridging of toner across the shorter diameter of the holes and clogging caused thereby are prevented.
The control electrodes may have a rough surface in a portion upstream of the holes in a direction in which the toner carrier moves. Thereby, electric fields are concentrated above the electrodes on the upstream side of the holes, activating the movement of the toner held on the toner carrier. Agglomeration and dispersion of toner are thereby repeated and toner density is made uniform.
The holes may be formed in a plurality of rows with a certain pitch space along a direction orthogonal to a direction in which the toner carrier moves, the holes and the control electrodes on a downstream side in the moving direction of the toner carrier having a diameter larger than that of the holes and the control electrodes on an upstream side. In this way, the amount of toner supplied through the holes on the downstream side is increased, making the toner density uniform, whereby image quality is enhanced.
An image forming device according to another aspect of the present invention includes: a toner carrier for holding and conveying charged toner; a toner passage controller including a plurality of holes through which toner is passed and a plurality of control electrodes surrounding the holes, control of passing of the toner through the holes being effected by applying voltage to the control electrodes in accordance with input image signals, the plurality of holes being formed in two rows in a zigzag fashion on an upstream side and a downstream side along a direction orthogonal to a direction in which the toner carrier moves; a plurality of connection electrodes respectively connected to each of the control electrodes for applying voltage thereto in accordance with the image signals, one group of connection electrodes connected to the control electrodes surrounding the holes on the upstream side row extending toward upstream side, while the other group of connection electrodes connected to the control electrodes surrounding the holes on the downstream side row extending toward downstream side; a plurality of dummy electrodes respectively connected to each of the control electrodes surrounding the holes on the downstream side row such as to extend toward upstream side; an image receiving member on which toner is deposited; and a backside electrode for attracting toner thereto disposed on the backside of the image receiving member.
According to the invention, the connection electrodes for the control electrodes provided in two rows and their respective driving ICs can be arranged with enough space on both upstream and downstream sides. The dummy electrodes are extended toward the upstream side from the control electrodes in the downstream side row in order to create electric fields whereby the toner amount supplied from the holes on the upstream and downstream sides is made uniform, eliminating the problem of white line noise. Also, the dummy electrodes cause the toner to repeatedly fly to and from the toner passage controller whereby the toner density is made uniform. Thus toner is efficiently and uniformly supplied from the toner carrier to the holes.
The plurality of holes may be arranged in a single or a plurality of rows along a direction orthogonal to a direction in which the toner carrier moves, and a plurality of electrodes may respectively be connected to each of the control electrodes such as to extend toward an upstream side in the moving direction of the toner carrier. The effects described above will likewise be achieved. The driving ICs for all of the control electrodes may be disposed on the upstream side, with all of the connection electrodes being extended toward the upstream side. Alternatively, part of the control electrodes may have their driving ICs on the downstream side, with their connection electrodes being extended toward the downstream side, while their dummy electrodes are extended toward the upstream side.
Alternatively, the plurality of holes may be arranged in a single or a plurality of rows along a direction orthogonal to a direction in which the toner carrier moves; and a plurality of connection electrodes may respectively be connected to each of the control electrodes for applying voltage thereto in accordance with the image signals such as to extend alternately toward an upstream side and a downstream side in the moving direction of the toner carrier; and a plurality of dummy electrodes may respectively be connected to the control electrodes from which the connect electrodes extend toward the downstream side such as to extend toward the upstream side.
The control electrodes may have a rough surface in a portion upstream of the holes in a direction in which the toner carrier moves. Thereby, electric fields are concentrated above the electrodes on the upstream side of the holes, activating the movement of the toner held on the toner carrier. Agglomeration and dispersion of toner are thereby repeated and toner density is made uniform.
The holes may be formed in a plurality of rows with a certain pitch space along a direction orthogonal to a direction in which the toner carrier moves, the holes and the control electrodes on a downstream side in the moving direction of the toner carrier having a diameter larger than that of the holes and the control electrodes on an upstream side. In this way, the amount of toner supplied through the holes on the downstream side is increased, making the toner density uniform, whereby image quality is enhanced.
An image forming device according to yet another aspect of the present invention includes: a toner carrier for holding and conveying charged toner; a toner passage controller including a plurality of holes through which toner is passed and a plurality of control electrodes surrounding the holes, control of passing of the toner through the holes being effected by applying voltage to the control electrodes in accordance with input image signals, the plurality of holes being arranged along a direction orthogonal to a direction in which the toner carrier moves; a plurality of deflection electrodes provided on both sides of each holes; an image receiving member on which toner is deposited; and a backside electrode for attracting toner thereto disposed on the backside of the image receiving member. According to this construction, the holes are arranged in one row, while toner is caused to be deposited on a plurality of points through a single toner passage hole by means of the deflection electrodes. Accordingly, the toner passage holes are arranged with a practicable pitch space, enabling a fine image to be formed. The single row arrangement of toner passage holes makes toner supply under constant conditions possible, whereby image quality is enhanced.
The deflection electrodes include longitudinal electrodes extending along the moving direction of the toner carrier and lateral electrodes extending perpendicular thereto from both sides of the longitudinal electrodes alternately at positions on an upstream side and a downstream side of each of the holes, so that one lateral electrode is shared by two adjacent holes. In this way, the deflection electrodes can be constructed simply and the pitch space between adjacent holes be made small.